Fluid streams derived from natural gas reservoirs, petroleum or coal, often contain a significant amount of acid gases, for example carbon dioxide, hydrogen sulfide, sulfur dioxide, carbon disulfide, carbonyl sulfide, hydrogen cyanide, ammonia, or mercaptans as impurities. Said fluid streams may be gas, liquid, or mixtures thereof, for example gases such as natural gas, refinery gas, hydrocarbon gasses from shale pyrolysis, synthesis gas, and the like or liquids such as liquefied petroleum gas (LPG) and natural gas liquids (NGL). Various compositions and processes for removal of acid gas contaminants are known and described in the literature. For example, it is well-known to treat such fluid streams with chemical solvents, such as amine solutions, which rely on a chemical reaction between the solvent and acid gas contaminants. The amine usually contacts the acidic gas contaminants in the fluid stream as an aqueous solution containing the amine in an absorber tower with the aqueous amine solution contacting the fluid stream counter currently. The regeneration of chemical solvents is achieved by the application of heat.
Alternatively, fluid streams may be treated with physical solvents, such as refrigerated methanol, dialkyl ethers of polyethylene glycols (DEPG), N-methyl-2-pyrrolidones (NMP), propylene carbonate, and the like which do not react chemically with the acid gas impurities. Physical solvents dissolve (absorb) the acid gas contaminants from the fluid stream, typically under high pressure. Since no chemical reactions are involved, physical solvent processes usually require less energy than chemical solvent processes. While the regeneration of chemical solvents is achieved by the application of heat, physical solvents can often be stripped of impurities by reducing the pressure without the application of heat. Physical solvents tend to be favored over chemical solvents when the concentration of acid gases or other impurities is very high. Unlike chemical solvents, physical solvents are non-corrosive, requiring only carbon steel construction.
Acid gas contaminants are removed by contacting the contaminated product gas with fresh solvent in an absorber or other specialized equipment operated under conditions of high pressure and/or low temperature which are favorable for the type of solvent used. Once the contaminants are removed, the decontaminated gas is ready for sale or for additional downstream conditioning, depending on the product stream specifications. The solvent is regenerated for reuse by driving off the absorbed contaminants under low pressure and/or high temperature conditions favorable for desorption. Flash tanks and/or stripper columns are typically used to effect this separation.
The formation of heat stable amine salts (HSAS) has long been a problem in chemical solvents containing amine solutions and formate based heat stable salts (FBHSS) in physical solvents used in gas conditioning. Heat stable salts (HSS) are called heat stable since they are not regenerable in the unit's stripping section. These HSS, such as amine salts of formate, acetate, glycolate, glyoxalate, oxalate, thiocyanate, thiosulfate, sulfate, sulfite and chloride, decrease the acid gas carrying capacity of the solvent and may increase solution viscosity, thus increasing unit operating costs and efficiency.
Several methods are disclosed to neutralize HSAS in amine solutions, for example see U.S. Pat. Nos. 5,622,681 and 5,912,387.
In general, HSAS are less of a problem for physical solvent acid gas removal processes as the solvent is not an amine. However, FBHSS are problematic for physical solvent acid gas removal processes treating a fluid stream containing ammonia as a contaminant. FBHSS may be formed between the ammonia and, for example, formates which are commonly present in low concentrations in physical solvents. If not counter acted, the FBHSS impurity can increase in concentration resulting in impaired efficiency and increased corrosiveness of the solution, which is particularly deleterious for carbon steel construction. Methods used to neutralize HSAS in chemical solvent processes are impractical if not impossible to apply to remove FBHSS a physical solvent process.
As such, it would be advantageous to have process wherein FBHSS are easily, economically, and efficiently removed from physical solvents so as to produce a regenerated physical solvent having an increased useful life and/or a less corrosive nature.